The warning shot to Mr Blair comes in a joint statement to be published in this week's Tribune newspaper. It will be debated at a London conference on 17 July on "Keeping the party Labour", which more than 90 Labour MPs have already promised to support.
Backing for the rebel declaration has increased since Labour's Euro election defeat this month, which provoked criticism that Mr Blair's relentless pursuit of middle-class voters had alienated the party's traditional supporters.
The statement has been signed by a wide cross-section of Labour MPs, including Peter Hain, the Welsh Office minister, who has already warned that the Government "appears as if it is being gratuitously offensive to its own natural supporters".
Other signatories include New Labour supporters such as Claire Ward and Maria Eagle; traditional right-wingers Gwyneth Dunwoody, Stuart Bell and Terry Davis; former ministers Mark Fisher and David Clark as well as left-wingers such as Tony Benn and Ken Livingstone.
The declaration says: "At the turn of the millennium, poverty and gross inequality still disfigure our society and destroy the life chances of millions across the globe.
"That challenge cannot be met by a reversion to 19th-century Liberalism out of whose failure Labour was born. Labour's future will be as a forward- looking, internationalist and democratic socialist party fighting for social justice into the 21st century."
One senior Labour MP said last night that the aim was to send a "shot across Blair's bows following the Euro elections debacle".
The critics want the Government to give much higher priority to tackling poverty. They want Mr Blair to abandon his goal - heavily influenced by his talks with Lord Jenkins of Hillhead, the Liberal Democrat peer - of creating a centre-left alliance of Labour and the Liberal Democrats to dominate the 21st century. They are alarmed by the joint declaration signed this month by Mr Blair and Gerhard Schroder, the German Chancellor, committing their governments to a low-tax, free- market economic policies.
Mark Seddon, a left-wing member of Labour's National Executive Committee and editor of Tribune, will warn in an editorial in Friday's issue: "The direction now favoured by both the Blair and Schroder governments will soon put the party on an absolute collision course with its natural supporters." He claims this view is shared by many due at next month's conference.
Mr Blair may decide to play down the rebel statement rather than attack it head on. "He regards the labels social democrat and social democracy as interchangeable," one aide said last night.
However, the thrust of the declaration is unmistakable and will be seen as a direct challenge to Mr Blair's authority. Another Labour MP said: "The numbers of people who have signed will send a powerful signal to Mr Blair. He must repudiate the Jenkins agenda for a centre-left coalition."
The Euro election defeat will be discussed today at the weekly meeting of the Parliamentary Labour Party. Although some Labour MPs remain jittery, party managers say nerves have been calmed by an opinion poll showing that Labour has retained its commanding lead over the Tories. The meeting will be told a summer fightback campaign will be launched in an attempt to reassure Labour supporters that the Government's policies will help the poor.
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